Boiler For Heat Bank

[Doctor Drivel]

MOD 2

post under your proper username and behave then i will leave it in

I am Doctor Drivel. I post on uk.d-i-y a lot.

They obviously have their very own uk.d-i-y Lunatic Association (affiliated). I'm sure affiliated to the DIYnot Lunatic Association (affiliated).

Do you arrange the chara banc meet up trips?

 

[Agile]

I think that I should remind you of the original advantage of a heat store before the days of condensing boilers.

A 200 litre store was heated to about 80°C and then directly heated the mains water to provide the DHW which was blended by the addition of cold water to give the say 60°C DHW to the point of use.

Because the 200 litres was maintained at such an elevated temperature it was able to give the equivalent of say 300 litres of water stored at a point of use temperature of 60°C.

With the advent of condensing boilers which are designed for a flow temperature of 70°C max the advantage of a thermal store is lessened if the condensing advantages are to be retained.

Whilst a thermal store can still be very useful it would need to be larger, in this example 300 litres, to be energy efficient. Since its only bought once every 10 years its not such a large expense to achieve the savings provided by condensing boilers.

Tony

 

[megawatt]

I'm not getting the Pandora for efficiency reasons Tony, for me it's about lots of hot water at mains pressure which I can't get with the current combi setup.

Taking the efficiency element out of the equation ... What boiler would you use?

 

[Bigburn]

Agile wrote

A 200 litre store was heated to about 80°C and then directly heated the mains water to provide the DHW which was blended by the addition of cold water to give the say 60°C DHW to the point of use.

This adds confusion .
The store cannot "directly" heat the mains cold.
This takes place through a heat exchanger which are most commonly immersed coils ,immersed plates heat exchangers or external plate heat echangers.

With the advent of condensing boilers which are designed for a flow temperature of 70°C max the advantage of a thermal store is lessened if the condensing advantages are to be retained.

Your comparing modern condensing boilers with outdated thermal storage systems. Modern stores have plate heat exchangers fitted which are very efficient at heat exchange. Take a look at some of the European systems .

Whilst a thermal store can still be very useful it would need to be larger, in this example 300 litres, to be energy efficient.

Can you clarify this more clearly please ?

Since its only bought once every 10 years its not such a large expense to achieve the savings provided by condensing boilers.

Do you mean the store is only purchased every 10 years or a condensing boiler is only purchased every 10 years.

 

[megawatt]

We're going off the point a bit here chaps as this discussion was done to death in the original post about whether or not to use a heat store.

Now that decision is made I'm simply after advice on what boiler to use to power both the heatbank and a 24kW sealed CH circuit.

 

[Agile]

The function of a boiler is the same, to heat water so any model would in theory be adequate.

However, the best choice can be quite important if you have different requirements to operate outside the usual parameters of heating systems.

Firstly, you probably need a boiler which has external flow temp controls, thats quite limiting.

If you are going to operate in non condensing modes much of the time then the non condencing efficiency may vary widely as the manufacturers concentrate on condensing efficiency.

If you do need high flow temperatures then some models are better than others, Worcester seem good in this respect but thats not a scientificic assessment. In normal use the efficiency is better if the flow temp is lower when the knob is in the "usual" full up!

Some models will not even give 70°C flow on heating because they modulate back if the heating load is high. A particular 28 kW boiler would only give 66°C flow rate when faced with a full 28 kW heating load because it was modulating back. Presumably the designers thought that it would always be used in an over specificied situation.

Tony

 

[Bigburn]

mega watt wrote

Now that decision is made I'm simply after advice on what boiler to use to power both the heatbank and a 24kW sealed CH circuit.

WB-Greenstar 24Ri

 

[croydoncorgi]

Hang on.....
You CAN'T use a Pandora with a SEALED system. It's open-vented (atmospheric pressure) only!!!

 

[esra_ptrap]

CroydonCorgi Wrote:

Hang on.....
You CAN'T use a Pandora with a SEALED system. It's open-vented (atmospheric pressure) only!!!

You certainly can

In the configuration the OP is using, the heatbank store water is open vented and un-pressurised and the Pandora is being used purely for HW provision via the 100kW heat exchanger.

The store is indirectly heated by the boiler on the sealed and pressurised CH circuit ... Usual setup with either S or Y plan controls.

This is a standard Pandora configuration.

 

[croydoncorgi]

My mistake - IMHO the Heatweb site is a tad confusing when determining exactly what goes with what!

But....

There MAY be a significant efficiency difference between ANY store where the entire contents of the store is circulated through the boiler, and ANY OTHER DESIGN involving an additional external heat-exchanger or coil inside the store (as in the Pandora). Potential problem is that the water that circulates through the HX or coil CANNOT transfer 100% of its heat to the store (basic thermodynamics), so the temperature of the Return water to the boiler will be higher than the temperature in the coolest part of the store. At lower temperatures, this may not have much effect on boiler efficiency but as the Return goes past 50 degrees, it certainly will.

DPS is obviously aware of this characteristic of condensing boilers - vented models can be configured with a mixing valve which optimises Return temperature. This feature cannot be included on the Pandora range. I've tried to find out from DPS how much difference it makes, without success so far.

Question is: do the benefits of a sealed system outweigh the costs of lost efficiency due to the configuration of the heatstore? Simple question, but no definite answer yet.

Given the number of damaged and badly-performing open-vented heating systems that I see every week, I'd prefer never to install another! With good installation practice and periodic maintenance, I've absolutely no doubts that sealed systems are more cost-effective long term than open-vented, with a lower total cost-of-ownership.

 

[esra_ptrap]

Agreed

 

[Agile]

"""With the advent of condensing boilers which are designed for a flow temperature of 70°C max the advantage of a thermal store is lessened if the condensing advantages are to be retained.


Your comparing modern condensing boilers with outdated thermal storage systems. Modern stores have plate heat exchangers fitted which are very efficient at heat exchange. Take a look at some of the European systems ."""

The point I was trying to get across is that a 200 litre store at 80°C holds about 15% more heat than a similar size store at only 70°C.

Therefore to hold more heat a larger store is needed. However good the thermal transfer rate is it cannot get more out of the store than it is storing.

Tony